Apologies for my ingnorance, but which one is or what is the Assist button?

__________________
05 X5 Diesel, Toledo Blue 19's, Panaramic Roof, Sportspack, Bluetooth & Tint...........
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive, well preserved body, but rather to skid in sideways - beer in hand, body thoroughly used up, screaming WOO HOO, what a ride!